1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device with a large screen having wide viewing angle characteristics
2. Description of the Related Art
In the prior art, there has been a display device utilizing electro-optical effects, such as a twisted nematic (TN) or super twisted nematic (STN) type liquid crystal display device. Now technology has been vigorously studied such that the viewing angle of such a liquid crystal display device may be enhanced.
An example of the technology for enhancing the viewing angle is seen in a TN type liquid crystal display device in which liquid crystal molecules are axially symmetrically aligned in each of liquid crystal regions separated by a wall of polymer (protrusion-like structures), namely, a so-called Axially symmetrically aligned Microcell (ASM) mode liquid crystal display device, as disclosed in Japanese Laid-Open Publication Nos. 6-301015 and 7-120728. A liquid crystal region surrounded by the polymer wall typically corresponds to a pixel region. In the ASM mode liquid crystal display device, liquid crystal molecules are axially symmetrically aligned, so that an observer recognizes less variation in contrast in any viewing directions; that is, wide viewing angle characteristics are obtained.
A production method of such an ASM mode liquid crystal display device is disclosed in Japanese Laid-Open Publication No. 7-120728. In accordance with the method disclosed in the publication, a protrusion-like structure is formed on a substrate in a grid pattern so that liquid crystal molecules are axially symmetrically aligned by the interaction between the protrusion-like structure and the liquid crystal molecules. Japanese Laid-Open Publication No. 10-133206 discloses another ASM mode liquid crystal display device in which the axially symmetrical alignment is achieved by the combination of a liquid crystal material of negative dielectric anisotropy (N-type liquid crystal material) and a vertical alignment layer.
A plasma addressed liquid crystal display device has potential as a large size liquid crystal display device and thus has been vigorously developed. An example of the plasma addressed liquid crystal display device is disclosed in Japanese Laid-Open Publication No. 1-217396. The plasma addressed liquid crystal display device includes a substrate, a thin dielectric sheet, ribs disposed between the substrate and the dielectric sheet, and a discharge channel (plasma channel) in the shape of a line surrounded by the substrate, the dielectric sheet and the ribs. The state of plasma discharge is changed by switching a voltage applied to noble gas filled in the discharge channel using an anode electrode and a cathode electrode. A liquid crystal layer is driven by a voltage applied between the discharge channel and a counter electrode, via the dielectric sheet.
The device disclosed in Japanese Laid-Open Publication No. 1-217396 is of the TN mode and therefore has a problem with its viewing angle characteristics. In order to solve this problem, Japanese Laid-Open publication Nos. 9-197384 and 10-186331 each disclose a plasma addressed liquid crystal display device of the above-described ASM mode.
However, the present inventors have found that the conventional ASM mode liquid crystal display device and the conventional ASM mode plasma addressed liquid crystal display device have the following problems (1) and (2).
(1) Reduction in Transmittance
FIG. 1 illustrates a part of the above-described ASM mode liquid crystal display device including a protrusion-like structure 96. A color filter layer 92, an overcoat layer 93 and a transparent electrode 94 are formed on a glass plate 90. The protrusion-like structure 96 and a column-like protrusion 98 are formed in a liquid crystal layer 95 which is disposed over those layers and the electrode. A counter substrate 99 is disposed on the column-like protrusion 98. When the protrusion-like structure 96 is black, i.e., light-blocking, and formed within a pixel region, an aperture ratio is significantly reduced and thus transmittance is decreased. When the protrusion-like structure 96 is transparent, the above-described problem does not arise. Nevertheless, the above ASM mode liquid crystal display device still has a disadvantage of a reduction in the aperture ratio, which will be described below with reference to FIG. 2. As schematically illustrated in FIG. 2, the liquid crystal layer 95 has a thickness of d1 between the transparent electrode 94 and the counter substrate 99, but a thickness of d2 directly above the protrusion-like structure 96 is smaller than d1. A portion of the liquid crystal layer 95 having the smaller thickness of d2 does not sufficiently contribute to display. In this case, when an N-type liquid crystal material is used, the state of display appears the same as when the reduction in the aperture ratio decreases the transmittance. When a P-type liquid crystal material (i.e., a liquid crystal material of positive dielectric anisotropy) is used, the contrast of display is reduced. This occurs for the following reason. The liquid crystal display device is designed using as a reference the thickness of d1 across the region having no protrusion-like structure 96. In the case where the thickness of d2 across the liquid crystal layer 95 directly above the protrusion-like structure 96 largely differs from d1, the retardation of such a region of the liquid crystal layer is deviated from the designed value, thus reducing the amount of light contributing to display.
(2) Slow Response Speed for a Gray Scale Image
In a conventional plasma addressed liquid crystal display device, a voltage is applied across a liquid crystal layer and a thin dielectric sheet (e.g., a glass sheet about 50 xcexcm thick). The voltage applied across the liquid crystal layer largely depends on the thickness of the liquid crystal layer. When the plasma addressed liquid crystal display device is of the ASM mode in which the above-described protrusion-like structure is used, a voltage applied across the liquid crystal layer directly above the protrusion-like structure is not sufficient since the thickness of such a region of the liquid crystal layer is thinner than the thickness of a region having no protrusion-like structure. Therefore, such a portion of the liquid crystal layer has a significantly slow response speed, reducing the entire response speed in displaying a gray scale image.
A liquid crystal display device according to the present invention includes a first substrate, a second substrate disposed facing the first substrate, a liquid crystal layer having liquid crystal molecules, interposed between the first and second substrates, and a groove structure having a plurality of grooves provided on the first substrate. The liquid crystal layer includes a plurality of liquid crystal regions defined by the plurality of grooves.
In one embodiment of the invention, the liquid crystal molecules in at least one of the plurality of liquid crystal regions are axially symmetrically aligned.
In one embodiment of the invention, the liquid crystal display device further includes a vertical alignment layer provided on a surface of at least one of the first and second substrates facing the liquid crystal layer. The liquid crystal layer includes a liquid crystal material of negative dielectric anisotropy.
In one embodiment of the invention, a side wall of the plurality of grooves is sloped with respect to a substrate surface.
In one embodiment of the invention, a slope angle of the sloped side wall is in the range of about 5xc2x0 to about 70xc2x0.
In one embodiment of the invention, a transparent electrode is provided between the liquid crystal layer and the groove structure.
In one embodiment of the invention, at least two of the liquid crystal regions correspond to a pixel region, and at least one of the grooves is provided in the pixel region.
In one embodiment of the invention, the plurality of grooves are disposed in a grid pattern.
In one embodiment of the invention, one of the first and second substrates is a plasma cell having a plasma channel for applying voltage to the liquid crystal layer.
In one embodiment of the invention, a product of a thickness d of the liquid crystal layer at the liquid crystal region and refractive index anisotropy xcex94n, i.e., dxc2x7xcex94n, is in the range of about 300 nm to about 600 nm.
In one embodiment of the invention, a polymer layer for stabilizing an alignment state of the liquid crystal layer is provided on the vertical alignment layer.
In one embodiment of the invention, the first and second substrates respectively have polarizers, the polarization axes of the polarizers provided on the first and second substrates are perpendicular to each other, and a compensation film is provided between at least one of the polarizers and the substrate including the at least one of the polarizers.
In one embodiment of the invention, the alignment directions of the liquid crystal molecules continuously vary in at least one of the plurality of liquid crystal regions.
In one embodiment of the invent ion, the alignment directions of the liquid crystal molecules continuously vary from within the groove to a region directly above the groove to an inward region of the liquid crystal region.
Hereinafter, functions of the present invention will be described.
In the liquid crystal display device according to the present invention, the grooves formed in the groove structure control the alignment state of the liquid crystal region in such a manner that continuously varies the alignment directions of the liquid crystal molecules. Thus, walls for separating the liquid crystal regions as used in the prior art are not necessary. Therefore, a reduction in transmittance can be thus prevented, and wide viewing angle characteristics are realized. Furthermore, the liquid crystal molecules within a groove region respond (change their orientation) prior to, and have influence on, the other portion of the liquid crystal region, unlike the liquid crystal molecules above the conventional protrusion-like structure, so that the apparent response speed becomes faster for producing a gray scale image.
When the liquid crystal molecules are axially symmetrically aligned in the liquid crystal region, wider viewing angle characteristics are realized.
When the vertical alignment layer is further provided on the surface facing the liquid crystal layer of the substrate, and the liquid crystal layer includes a material of negative dielectric anisotropy, display having a high degree of contrast can be realized in a normally black mode.
When the side wall of the groove of the groove structure is sloped, the orientations of the liquid crystal molecules aligned perpendicular to the side wall are effectively controlled in the presence of an applied voltage. The alignment control of the liquid. crystal molecules is stabilized, particularly when the slope angle of the sloped side wall is in the range of about 5xc2x0 to about 70xc2x0.
When the transparent electrode is provided between the groove structure and the vertical alignment layer, the groove structure does not cause a decrease in a voltage between the transparent electrode and the liquid crystal layer, so that the driving voltage for the liquid crystal layer can be reduced.
Even when the groove is provided in the pixel region, the liquid crystal molecules within the groove contribute to display, so that the transmittance is not reduced. Accordingly, a large pixel region can include, for example, a plurality of the liquid crystal regions, and thus the response speed of the liquid crystal layer can be fast.
When the grooves are disposed in a grid pattern, the liquid crystal molecules in the liquid crystal layer can be driven in the axially symmetrical mode while the liquid crystal region surrounded by the grooves is in register with the pixel region.
When the plasma substrate is used in the liquid crystal display device of the present invention, the device can be driven in the plasma addressed mode. In this case, the liquid crystal layer has a larger thickness at the groove region where a larger voltage is applied to the liquid crystal molecules than the other regions. In particular, the plasma addressed liquid crystal display device can be effectively driven by a low voltage.
When the retardation of the liquid crystal layer, which is a product of the thickness d of the liquid crystal layer and the refractive index anisotropy xcex94n, is in the range of about 300 nm to about 600 nm, a satisfactory contrast is obtained.
When a polymer layer for stabilizing the alignment state of the liquid crystal layer is formed on the vertical alignment layer using a polymer stabilizing technique, the stabilization of the alignment state is further ensured.
When a pair of substrates have polarizers in such. a way that their polarization axes are perpendicular to each other, and at least one compensation film having negative refraction index anisotropy is provided between either pair of the substrates and the polarizer, the viewing angle is further improved.
Thus, the invention described herein makes possible the advantages of providing a liquid crystal display device having a high degree of transmittance, a fast response speed for producing a gray scale image, and a wide viewing angle.